A Mobile Station (MS) may have multiple physical antennas and for each physical antenna there is an associated transmit power amplifier. From each physical antenna a Sounding Reference Signal (SRS) is transmitted with the purpose of enabling measurements at a receiving Base Station (BS). SRSs from different antennas may be code, frequency or time multiplexed, and furthermore SRSs must be multiplexed between all users in a cell. The SRS resources are allocated to different MS in a semi-static fashion by higher layer signalling from a BS to a MS.
Furthermore, to enable coherent demodulation of transmitted signals at a receiving BS Demodulation Reference Signals (DRSs) are used. These reference signals define different antenna ports and are code multiplexed between antenna ports and users. Mapping of antenna ports to physical antennas is a method known in the art and an example of such mapping is shown in FIG. 1. The size of a common set of available reference signals is restricted by the number of antenna ports.
The number of available SRSs is much larger than the number of available DRSs since SRSs are multiplexed in time and frequencies as well as in code dimensions, whereas DRSs are multiplexed in code dimensions.
An uplink cellular wireless communication system with multiple users is considered. To estimate channels from each user to a receiver, and to be able to perform coherent demodulation of transmitted information from the users DRSs are transmitted, wherein the DRS from each antenna and each user are orthogonal to each other. It may be noted that another term for DRSs is pilot signals for demodulation.
Furthermore, assume that the size of a set of available orthogonal reference signals (such as DRSs) is finite and limited. It means that the number of simultaneously multiplexed users will be upper limited by the size of the set of orthogonal reference signals and the upper limit is obtained if all users use a single orthogonal reference signal. Further, with the introduction of multiple physical antennas at the user terminals, it is possible to use transmit diversity or spatial multiplexing to enhance the capacity or coverage of the system at the expense of an increased number of used reference signals per user.
Transmit diversity will give an increased Signal-to-Noise Ratio (SNR) at a receiver. The increased SNR will extend the coverage of a transmitter-receiver link or makes it possible to use a more aggressive code rate and/or modulation scheme so that a larger number of information bits can be transmitted. This is denoted link adaptation and will increase the capacity of the link. Sometimes link adaptation is not utilized for a certain channel, but instead a fixed modulation and code rate is selected based on the SNR for a worst case user terminal. The use of transmit diversity for this channel is therefore unnecessary for the terminals which already have a medium to high received SNR. Transmit diversity will in this case only marginally improve the detection performance. An example of a fixed modulation and code rate is transmission of ACK/NACK in a 3GPP LTE system, which is not adapted to the link quality, but merely designed for a worst possible encountered SNR. If a MS close to a BS (with low path loss) transmits an ACK/NACK message, the BS will experience a high SNR without the use of transmit diversity, and will correctly decode the ACK/NACK message with very high probability. The extra gain in SNR provided by transmit diversity is therefore unnecessary for this particular user. If transmit diversity over multiple antenna ports is used in this case, it will only result in consumption of orthogonal reference signals from the set of orthogonal reference signals but the benefit is very small or even none existing.
Spatial multiplexing increases the spectral efficiency by transmitting independent data streams from each antenna. If channels from the antennas are correlated, the decoding of the data streams will have poor performance and the advantage of multiple stream spatial multiplexing is thereby limited. The reason is that the correlated channel does not have the ‘richness’ to support multiple streams. Hence, the use of multiple antennas, wherein each antenna would use a DRS from the common set of DRSs is unnecessary and will not provide higher spectral efficiency.
If the number of allocated DRSs for a user is high, as discussed in the two cases above, then the common set of DRSs will be consumed quickly and as a consequence, the maximal number of multiplexed users will be low. FIG. 2 shows an example of how DRSs may be allocated for four MSs with two antennas each.
According to prior art a method is proposed where a receiver measures signals transmitted from a transmitter with multiple antenna ports, and feeds back a channel quality indicator to the transmitter containing at least a preferred transmission mode. The transmission mode consists of a transmission rank and a precoding matrix index, where the rank is the number of parallel spatial streams to be transmitted. The transmitter then subsequently provides an adaptation of the transmission mode based on the reported channel quality indicator. The adaptation of the transmission mode is made using a precoding matrix, modulation-coding scheme and the rank. The number of antenna ports (or equivalently the number of reference signals) in this method is constant and the number of parallel transmitted streams is adapted using a rank feedback and a precoder matrix.
In another solution according to the prior art, a downlink transmission scheme is proposed where a Common Reference Signal (CRS) is transmitted from each antenna port. Hence, there is no set of reference signals that needs to be shared between users in the system. Therefore, the same CRS are utilized by all users. For downlink transmit diversity in this prior art solution all antenna ports are always used and the transmit diversity method that matches the number of antenna ports is selected. For downlink spatial multiplexing in this case, the CRSs are used to calculate the rank which determines the number of multiplexed spatial layers of transmission the downlink channel can support. The user terminal then explicitly signals to a BS its desired rank. For example, if the rank is measured to be one then a precoding vector will be used to map the single data stream to the multiple antenna ports. The user will use the CRS for all antenna ports and the knowledge of the precoding vector to demodulate the transmitted signal.
Hence, there is a need in the art for efficient use of reference signals in the uplink of a wireless communication system.